1. Field of the Invention
The present invention relates to a pixel circuit of a light emitting element, particularly of an organic electro-luminescence (EL) element to be used in an organic EL display panel, and a drive method thereof, and more particularly relates to a light emitting element circuit which supplies an electric current to the organic EL element through a current mirror circuit and a drive method thereof.
2. Description of the Related Art
In recent years, an electron element with the use of an organic semiconductive material has widely been developed, and it is reported that an organic EL (Electro-Luminescence) element of a light emitting element, an organic TFT (Thin Film Transistor) and an organic solar cell have been developed. Among them, the organic EL display is considered to be promising as a technique which is likely to be practically used in the nearest future.
A structure of the organic EL display panel is classified into a passive-matrix type and an active-matrix type. The passive-matrix type is based on an impulse operation, so that an organic EL element using the passive-matrix type hardly provides a display panel with high brightness because the organic EL element has a trade-off relation between brightness and the life. On the other hand, the active-matrix type does not necessarily need the impulse operation but can make the organic EL element almost uninterruptedly stay on, and accordingly is effective for extending the life of the organic EL element. However, the active-matrix type has a big problem that the characteristics of a transistor (thin film transistor) and the organic EL element composing a pixel circuit and a peripheral circuit are non-uniform and fluctuate with time.
A voltage programming method proposed in U.S. Pat. No. 6,229,506 is a method of compensating a variation of a threshold and drift of the thin film transistor by using an auto zero control.
A current programming method proposed in U.S. Pat. No. 6,373,454 is also a method of compensating the variation of the threshold and mobility of the thin film transistor.
U.S. Pat. No. 6,501,466 proposes a pixel circuit with the use of a current mirror circuit. The current mirror circuit can separate a circuit for supplying an electric current to the organic EL element from a selection circuit, accordingly can decrease the power consumption while performing a current program, and is particularly effective when a thin film transistor having a weak driving force (small mobility) is used.
Leaflet of WO 2005-029455 proposes a pixel circuit with the use of another current mirror circuit. The pixel circuit can compensate the characteristic drift of a thin film transistor and an organic EL element by flowing an electric current to the organic EL element even during programming. The pixel circuit can effectively compensate the characteristic drift even when the thin film transistor has incomplete saturation characteristics and cannot function as a constant current source.
The current mirror circuit has many advantages, but has a defect that the same current cannot be held when the characteristics of two thin film transistors composing a current mirror are deviated from each other. When the temporal cumulative amounts of electric currents flowing through the two thin film transistors are different, the characteristic drifts become different, and as a result, the characteristics of the thin film transistors are greatly deviated from each other after having been operated for many hours.
FIG. 14 is a current mirror type of a pixel circuit which is proposed in the above described leaflet of WO 2005-029455. In the current mirror type of the pixel circuit, one thin film transistor of the two thin film transistors composing the current mirror flows an electric current for current programming and the other thin film transistor drives an organic EL element. Accordingly, a current and voltage stress (which is a force that causes a characteristic change and is used hereafter in the present specification) applied to the two thin film transistors composing the current mirror is greatly different, which causes a problem that the above described difference of the characteristics between the thin film transistors is formed.
Thus, it is desired for the organic EL element having the current mirror type of the pixel circuit to suppress the deviation of the characteristics between the two thin film transistors composing the current mirror.